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Normal for a huge vacuum when removing my gas cap?!

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  • Normal for a huge vacuum when removing my gas cap?!

    Whenever i go to fill up at the gas station and i remove my gas gap, there is a fairly strong hissing, as if my fuel tank is under a large vacuum. is this normal for DA tegs?

  • #2
    How much gas is in the tank when you fill it up

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    • #3
      this happened to me too. it was as though the tank was under a lot of pressure. i thought the cap was going to shoot at me when i took it off. a little hiss is normal but this is ridiculous...

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      • #4
        Just remember when your at a full tank there's not as much air in the tank then when your at a lower fuel level. That being said when the gas cap is being removed, the hissing sound is air being pulled back into the tank.. the cap "shooting off" sounds like it had set in a warmer area and the fuel molecules expanded pitting a force on the inside of the tank. An when you removed the cap it released the pressure. And well you was its target lol.

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        • #5
          The hissing is pressure being released from the tank (not pulled in^^). This is normal, It means your fuel system holds pressure like it's supposed to. No sounds would be more concerning.

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          • #6
            ^ Yup. Your fuel system pressure is working properly. I usually get a strong pressure when my tank is near empty. I slowly take off the cap to release the pressure. Not sure if there's any benefits compare to just pulling the cap off quick.

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            • #7
              How's it releasing pressure in a sealed system.

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              • #8
                is that a joke? theres gonna be pressure especially in a sealed system

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                • #9
                  Your fuel tank IS under vacuum pressure... What else do you think your fuel pump is doing? Heh

                  Virtually any fuel tank will make a vacuum noise if it has sat in the sun all day, or has been ran from Full to Empty..

                  Has happened on every car I've owned.

                  Both ways mentioned are true (afaik).. if it has sat in the sun, pressure is built up and pressure will escape the tank when you remove the cap.

                  But if you run from Full to Empty, the tank will have vacuum built up and suck air in when you take the cap off.

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                  • #10
                    The issue for me isnt the hiss sound. Im fully aware how normal this is. My concern is how much pressure was released when i removed my cap. It was a VERY large amount. Way more than i had ever heard/felt on any car. I'll post a video next time.

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                    • #11
                      Thanks for backing me unified112

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                      • #12
                        the fuel system is not completely sealed. if it were, the tank would collapse in on itself through vacuum from the displaced volume of fuel. likewise, a full tank on a hot day could bust it open from pressure. the fuel tank is supposed to breathe through the evap system. specifically, the canister. the can is vented to the intake boot. because it's past the filter, it vents clean air. the cap is also vented only as a precautionary measure. like if the tank evap venting fails. however, the cap is set to vent only at extreme pressure. OP, if you're getting high vacuum through the cap when it's opened, check the evap system lines. that is, the line from the tank to the can, and the can vent line that is plumbed into the intake boot. make sure they aren't obstructed or clogged.
                        Last edited by welfare; 16 Dec 2012, 01:10:44.

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                        • #13
                          Originally posted by DA_all_day View Post
                          The issue for me isnt the hiss sound. Im fully aware how normal this is. My concern is how much pressure was released when i removed my cap. It was a VERY large amount. Way more than i had ever heard/felt on any car. I'll post a video next time.

                          My Teg is the loudest of any car I've ever owned as well. Who knows why. I doubt it's an issue. Do you fill all the way up and go to empty?

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                          • #14
                            i dont think its an issue either, just saying its weird but no. i dont usually go put a full tank. maybe a little over half and run it down until the light comes on.

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                            • #15
                              I usually let it get fairly low, but the sound is much louder than any car i've owned. I was just worried about pulling excessive vacuum in the tank could strain the pump, but i wasn't sure if the tank was under vacuum or pressure. Just in case, i'll look at the evap. Thanks!

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